REACTIVE POWER CONTROL FOR SINGLE-PHASE GRID-TIE INVERTERS USING QUASI SINUSOIDAL WAVEFORM
Abstract
The paper presents a reactive power controltechnique for single-phase Photovoltaic (PV) inverters, especiallyunfolding inverters. The proposed system retains the benefit of theunfolding inverters having low material cost and semiconductorlosses, and tackles the drawback of the standard unfoldinginverter not having capability of reactive power injection. It isimportant to note that reactive power delivery is mandatory forPV inverters according to the recent announced regulations. Theconcept is based on changing the shape of the grid currentwaveform but keeping the same zero crossing points as in the unitypower factor condition. The current waveform is governed by realpower and reactive power, at the price of an acceptabledeformation. The operating principles of the proposed techniqueand mathematical derivations of the grid current function areprovided in the paper. Experimental results in a grid-tie inverterprototype have shown a good agreement with the derived theory,and they confirm the feasibility of using the proposed technique ingrid-tie inverters.
CONCLUSIONS
This paper presented a control technique for single phase grid-tie inverters. The control technique allows reactive powerinjection to unfolding topologies, which were limited to unitypower factor operation. The idea was to provide a QuasiSinusoidal Waveform as a grid current reference to injectreactive power. The mathematical models were provided andexplained. A prototype of a unipolar switching full-bridgeinverter was built and evaluated for the QSW technique.Working principle of new controller was explained. Bycomparing measured values and theoretical values,experimental results showed a good agreement with the theory.It was shown that reactive power injection is possible bygenerating quasi sinusoidal waveform current through theinverter, without changing any hardware components.
EXISTING SYSTEM:
It is well-known that SCR are turned off at zero current with relatively long commutation time which makes difficult for the injection of reactive power. Thus, the impossibility of injecting D. Li, C.N.M. Ho (Corresponding author) and L. Luo are with the RIGAreactive power is the main drawback of unfolding inverters. However, a new regulation has been recently published torequire that PV inverter products must have the capability toadjust Power Factor (PF) up to 0.95 in either inductive orcapacitive modes. In consequence, a lot of alreadydesigned commercial products are facing the problem of notpassing such a regulation. Some advanced modulation methodshave been proposed recently to satisfy the PF regulation, but they cannot be applied to unfolding inverters since thezero crossing points of grid voltage and current are not the same. Thus, manufacturers have to redesign PV inverter systemcompletely using different semiconductors or changingtopology to be more complicated and expensive such as NeutralPoint Clamped (NPC) inverters to satisfy the regulation in theirfuture products.
PROPOSED SYSTEM:
This paper follows up the idea in the literature topropose a control technique for grid-tie inverters, especiallyunfolding inverters. The delivered grid current is modified to theproposed Quasi Sinusoidal Waveform (QSW) to carry reactivepower, and thus satisfying the PF regulations. The modifiedcurrent keeps the same zero crossing points as the grid voltage.The injection of reactive power is thus possible at the expensesof an acceptable current shape deformation. Although it containsharmonics during reactive power delivering and it is thedrawback of the proposed waveform, the total harmonicdistortion (THD) is relatively low. Recall that, during unitypower factor, the current satisfies the THD regulation such asIEC61000-3-2 [20]. This paper provides the mathematicalderivation of the proposed grid current reference function.Numerical and experimental results of the proposed techniqueapplied to an grid-tie inverter are presented to verify thetheoretical findings.In this paper, Quasi Sinusoidal Waveform (QSW) is definedas a waveform which is formed by sinusoidal waveforms withdifferent frequencies. The resultant waveform is close to asinewave but it includes harmonics in the signal showsthe proposed QSW signal indicated by a solid blue line, whichis proposed as the reference for the delivered grid current. Thiswaveform is divided into 4 time intervals, where the boundariesare fixed at peaks and zero crossings of this waveform. Noticethat each interval consists of a fraction of a sine function thatfrequency alternates between two different values.
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